A Manhattan fundraiser headlined by former Secretary of State Hillary Clinton Monday night is expected to generate more than $1 million for Democrat Bill de Blasio’s campaign for New York City mayor, a person familiar with the matter said.

The cash haul will be the largest single infusion of money for Mr. de Blasio, the city’s public advocate, since he launched his mayoral campaign earlier this year. Mr. de Blasio, 52, holds a 40-plus percentage point lead in public polling and is widely expected to be elected the city’s 109th mayor on Nov. 5.

Mr. de Blasio, who served as Mrs. Clinton’s campaign manager during her successful bid for the U.S. Senate in New York in 2000, was endorsed by former President Bill Clinton and Mrs. Clinton shortly after he clinched the Democratic nomination last month. President Barack Obama has also endorsed Mr. de Blasio.

“Look, tonight, to me, is a very, very special moment,” said Mr. de Blasio, when asked at a news conference Monday morning about the fundraiser.